Selectable dual position magnetron

ABSTRACT

A dual-position magnetron that is rotated about a central axis in back of a sputtering target, particularly for sputtering an edge of a target of a barrier material onto a wafer and cleaning material redeposited at a center of the target. During target cleaning, wafer bias is reduced. In one embodiment, an arc-shaped magnetron is supported on a pivot arm pivoting on the end of a bracket fixed to the rotary shaft. A spring biases the pivot arm such that the magnetron is urged towards and overlies the target center. Centrifugal force at increased rotation rate overcomes the spring bias and shift the magnetron to an outer position with the long magnetron dimension aligned with the target edge. Mechanical stops prevent excessive movement in either direction. Other mechanisms include linear slides and actuators.

RELATED APPLICATION

This application claims benefit of provisional application 60/555,992,filed Mar. 24, 2004.

FIELD OF THE INVENTION

The invention relates generally to sputtering of materials. Inparticular, the invention relates to the magnetron creating a magneticfield to enhance sputtering.

BACKGROUND ART

Sputtering, alternatively called physical vapor deposition (PVD), is themost prevalent method of depositing layers of metals and relatedmaterials in the fabrication of integrated circuits. Sputtering wasoriginally used to deposit generally planar layers of material on awafer and was particularly used for depositing aluminum electricalinterconnect lines. However, in recent years, the emphasis andchallenges have shifted to depositing materials used in verticalinterconnections in high aspect-ratio vias and similar verticallyoriented structures formed in and through dielectric layers. Coppermetallization has further changed the emphasis since bulk copper can beeasily deposited by electrochemical plating (ECP). However, various thinliner layers are required prior to the ECP, for example, barrier layers,such as Ta and TaN, to prevent the copper from migrating into the oxidedielectric and copper seed layers to provide a plate electrode and toinitiate the growth of the copper ECP layer.

Techniques have been developed to allow the sputter deposition of thinuniform layers onto the walls of high aspect-ratio holes. One suchtechnique that has met significant commercial success is self-ionizingplasma (SIP) sputtering in which a significant fraction of the sputteredatoms are ionized and thus can be electrostatically attracted deepwithin narrow holes. It is called self-ionizing plasma because some ofthe sputtered ions are attracted back to the sputtering target tosputter yet more atoms or ions, thereby reducing the need for an argonworking gas and allowing sputtering at a lower pressure. The extreme ofSIP is sustained self-sputtering (SSS) in which the sputtered ions aresufficient to maintain the sputtering plasma and hence the argon can beremoved.

A conventional PVD chamber 10, with a few modifications for SSS or SIPsputtering, is illustrated schematically in cross section in FIG. 1. Theillustration is based upon the Endura PVD Reactor available from AppliedMaterials, Inc. of Santa Clara, Calif. The chamber 10 includes a vacuumchamber body 12 sealed through a ceramic insulator 14 to a sputteringtarget 16 having at least a front face composed of the material, usuallya metal, to be sputter deposited on a wafer 18 held on a heater pedestalelectrode 20 by a wafer clamp 22. Alternatively to the wafer clamp 22, acover ring or an electrostatic chuck may be incorporated into thepedestal 20 or the wafer may be placed on the pedestal 20 without beingheld in place. The target material may be aluminum, copper, aluminum,titanium, tantalum, cobalt, nickel, molybdenum, alloys of these metalscontaining less than 10 wt % of an alloying element, or other metals andmetal alloys amenable to DC sputtering. On the other hand, RF sputteringmay be used to sputter material from a dielectric target. A groundedshield 24 held within the chamber body 12 protects the chamber wall 12from the sputtered material and provides a grounded anode. A selectableand controllable DC power supply 26 negatively biases the target 14 toabout −600VDC with respect to the shield 24. Conventionally, thepedestal 20 and hence the wafer 18 are left electrically floating, butfor most types of SIP sputtering, an RF power supply 28 is coupled tothe pedestal 18 through an AC capacitive coupling circuit 30 or morecomplex matching and isolation circuitry to allow the pedestal electrode20 to develop a DC self-bias voltage in the presence of a plasma. Anegative DC self-bias attracts positively charged sputter ions createdin a high-density plasma deeply into a high aspect-ratio holescharacteristic of advanced integrated circuits. Even when the pedestal20 is left electrically floating, it develops some DC self-bias.

A first gas source 34 supplies a sputtering working gas, typicallyargon, to the chamber body 12 through a mass flow controller 36. Inreactive metallic nitride sputtering, for example, of titanium nitrideor tantalum nitride, nitrogen is supplied from another gas source 38through its own mass flow controller 40. Oxygen can alternatively besupplied to produce oxides such as Al₂O₃. The gases can be admitted fromvarious positions within the chamber body 12. For example, one or moreinlet pipes located near the bottom of the chamber body 12 supply gas atthe back of the shield 24. The gas penetrates through an aperture at thebottom of the shield 24 or through a gap 42 formed between the coverring 22 and the shield 24 and the pedestal 20. A vacuum pumping system44 connected to the chamber body 12 through a wide pumping port 46maintains the interior of the chamber body 12 at a low pressure.Although the base pressure can be held to about 10⁻⁷ Torr or even lower,the conventional pressure of the argon working gas is typicallymaintained at between about 1 and 100 milliTorr. However, forself-ionized sputtering, the pressure may be somewhat lower, forexample, down to 0.1 mTorr. For sustained self-sputtering, particularlyof copper, once the plasma has been ignited, the supply of argon may bestopped, and the chamber pressure may be made very low. A computer-basedcontroller 48 controls the reactor including the DC power supply 26 andthe mass flow controllers 36, 40.

When the argon is admitted into the chamber, the DC voltage between thetarget 16 and the shield 24 ignites the argon into a plasma, and thepositively charged argon ions are attracted to the negatively biasedtarget 16. The ions strike the target 16 with a substantial energy andcause target particles to be sputtered from the target 16. Some of thetarget particles strike the wafer 18 and are thereby deposited on it,thereby forming a film of the target material. In reactive sputtering ofa metallic nitride, nitrogen is additionally admitted into the chamberbody 12, and it reacts with the sputtered metallic atoms to form ametallic nitride on the wafer 18.

To provide efficient sputtering, a magnetron 50 is positioned in back ofthe target 16. It includes opposed magnets 52, 54 coupled by a magneticyoke 56 to produce a magnetic field within the chamber in theneighborhood of the magnets 52, 54. Typically in SIP sputtering, themagnetron 50 is small, nested, and unbalanced with one or more innermagnets 52 surrounded by opposed outer magnets 54 of greater magneticintensity. The magnetic field traps electrons and, for chargeneutrality, the ion density also increases to form a high-density plasmaregion 58 within the chamber adjacent to the magnetron 50. To achieveuniform sputtering onto the wafer 18, the magnetron 50 is usuallyrotated about the center 60 of the target 16 by a shaft 62 driven by amotor 64. Typical rotation speeds are 50 to 100 rpm. In a conventionalmagnetron, the shaft 62 is fixed with respect to the magnets 52, 54 andis coincident with the target center 60 so that the magnetron 50 sweepsa constant track about the target center 60.

Fu in U.S. Pat. No. 6,306,265 discloses several designs of a magnetronuseful for SSS and SIP. For these applications, the magnetron shouldproduce a strong magnetic field and have a small area. Rotating themagnetron can nonetheless provide uniform sputter deposition and fulltarget erosion if desired. The magnetron should include an inner poleassociated with one or more inner magnets 52 surrounded by a continuousouter pole of the opposite polarity associated with the outer magnets54. The inner and outer poles are unbalanced in the sense that the totalmagnetic flux produced by the outer pole is substantially greater thanthat produced by the inner pole by a factor of at least 1.5. Thereby,magnetic field lines from the outer pole 54 extend deeply into thechamber towards the wafer 16. The power supplied by the DC supply 26 tothe target 16 should be high, of the order of 20 kW for a 200 mm wafer.However, scaling the power supply for 300 mm wafers presents somedifficulties. Nonetheless, the combination of high power and smallmagnetron area produces a very high power density beneath the magnetron50 and hence a moderately high-density plasma area 58 without the use ofsupplemental plasma source power, such as would be provided by RFinductive coils. The form and size of the magnetron 50 are related tosome aspects of the invention.

To counteract the large amount of power delivered to the target, theback of the target 16 may be sealed to a backside coolant chamber 66.Chilled deionized water 68 or other cooling liquid is circulated throughthe interior of the coolant chamber 66 to cool the target 16. Themagnetron 50 is typically immersed in the cooling water 68, and thetarget rotation shaft 62 passes through the back chamber 66 through arotary seal 70.

Such an SIP chamber 10 can be used for both sputtering the barrierlayer, for example, of TaN/Ta from a tantalum target and sputtering thethin copper seed layer from a copper target. Particularly for barrierlayers, continuous and symmetrical deposition with the structure iscritical for minimum sidewall coverage requirement and via bottomthin-down/punch-through process. Barrier sputtering has been found to beoptimized for relatively small magnetrons that concentrate on theperipheral region or edge of the target with little or no effectivesputtering in the target center. Material eroded from the targetperiphery region possesses preferred incident angles to achievesymmetrical step coverage. In addition, the small magnetron produces ahigh power density and hence high ionization fraction with a relativelylow DC power supply. However, target erosion at the periphery of thetarget will produce re-deposition around the central area of the target,which central area is not on net eroded. The re-deposition needs to beeliminated during the sputtering or cleaning process. The cleaningprocess will be described below. Uniform target erosion and high averagesputtering rate in the case of Cu and Al deposition are not primaryconsiderations for the small amount of material being sputtered for thethin barrier or seed on each wafer.

For SIP sputtering with magnetrons not extending over an entire radiusof the target 16, the rotating magnetron 50 does not scan the entirearea of the target 16 and sputtered material tends to redeposit on thenon-scanned areas. Copper redeposition does occur but is not generallyconsidered to be a significant problem since the redeposited copperbonds relatively well with the copper target. Barrier redeposition,however, may present a significant problem. Part of the barriersputtering may occur in a nitrogen ambient in a process called reactivesputtering to deposit a metal nitride layer, such as TaN or TiN, on thewafer. The nitride also redeposits on the metal target and grows inthickness over many wafer cycles. Such redeposited material is prone toflaking and hence presents a source of particles. As a result, it isoften considered necessary to prevent the redeposited barrier materialfrom flaking, preferably by preventing its growth beyond a criticalthickness.

Rosenstein et al. (hereafter Rosenstein) in U.S. Pat. No. 6,228,236 haspresented a solution for the redeposited material. They affix theirmagnetron 50 to an eccentric arm such that centrifugal force can becontrolled to cause the magnetron to assume two radial positionsdepending upon the rotational direction of the magnetron drive shaft 62.Rosenstein effectively interposes a radial translation mechanism 74between the rotation drive shaft 62 and the magnetron 50. He isprimarily concerned with redeposition on the target periphery outsidethe operational area of the target. His design provides a small radialtranslation of the magnetron and the orientation of their magnetron tothe rotation circumference remains substantially unchanged between thetwo positions. Also, the switching depends at least in part onhydrodynamics. The Rosenstein design could be modified to enablecleaning the target center, but this would rely on a reversal of themagnetron rotation. It is instead desired to provide a magnetron designwhich avoids the need to reverse the direction of rotating themagnetron, thereby quickening the transition between multiple radialrotation diameters. Such a design would desirably minimize the timenecessary for achieving a clean mode position so as to maximize thefraction of time for wafer deposition, in order to obtain highthroughput.

Various types of planetary magnetrons have been proposed, see forexample U.S. Pat. application Ser. No. 10/152,494, filed May 21, 2002 byHong et al. and now issued as U.S. Pat. No. 6,852,202, and U.S. patentapplication Ser. No. 10/418,710, filed Apr. 17, 2003 by Miller et al.and now issued as U.S. Pat. No. 6,841,050, both of which are commonlyassigned with the present application. The disclosed planetarymechanisms are capable of scanning a small magnetron over substantiallyall of the target surface in a planetary path produced by two rotationalarms. Although planetary scanning in a single though convolute path canbe used to avoid redeposition, it is nonetheless often desired toconfine the primary sputtering to a relatively narrow radial range ofthe target.

SUMMARY OF THE INVENTION

In one aspect of the invention, a two-step sputter process includes oneor more steps of sputter depositing a barrier metal on a substrate whilescanning the outer edge of a target with a small magnetron moving in anouter circular path and another step of cleaning the target by movingthe magnetron towards the center of the target and scanning at least thecenter of the target with the magnetron moving in an inner circularpath. The cleaning may be performed for every substrate being depositedor may be performed after every few substrates or every few hundreds ofkilowatt hours of target power.

A mechanism moves the effective center of the magnetron with respect tothe target radius and allows a rotary shaft to rotate the magnetronabout the target center while at different target radii.

One unidirectional, multi-speed centrifugal mechanism for adual-position magnetron supports the magnetron on a pivot arm pivotingon a bracket which rotated about the central axis of a sputter target bya mechanical drive shaft. A spring or other bias means biases themagnetron in one radial direction with respect to the central axis.Centrifugal force dependent upon the rotation rate of the drive shaftcan be set high enough to overcome the bias force. Thereby, selection ofthe rotation rate causes the magnetron to rotate at different radii fromthe central axis. Preferably, a tension spring connected between thebracket and the pivot plate biases the magnetron towards central axiswhile centrifugal force on the magnetron urges it outwardly.

Mechanical stops may be used to prevent over pivoting in eitherdirection, thereby providing definite control over the rotation radius.The stops should be shock absorbent, such as resilient buffers or shockabsorbers.

The pivot mechanism between the bracket and pivot arm may include twowater-sealed bearings including at least one dynamic seal to allow thedual-position magnetron to operate in a cooling water bath at the backof the target.

A centrifugal dual position mechanism can alternatively be implementedin a linear slider in which a radially extending slot is formed in therotating bracket. A support for the magnetron is fit in the slot and canslide radially therein. One or more springs may be used to bias themagnetron towards the rotary center while centrifugal force urges themagnetron away from the center. Engagement of the support with theeither end of the slot provides a positive mechanical stop.

A pivoting motion of the magnetron, whether through centrifugal force orwith an actuator, is advantageously applied to an elongated magnetronhaving a short dimension and a long dimension. For edge sputtering ofthe target, the magnetron is located near the target edge with the longdimension perpendicular to a radius from the rotation center. For centercleaning, the magnetron is located nearer the target center with itslong dimension inclined at a smaller angle to the radius from therotation center, for example, at less than 60°.

Externally controlled actuators may be used to selectively move amagnetron in a direction at least partially radially of a rotating plateon which the magnetron is supported. For example, a liquid or pneumaticactuator located on the plate and driven by a fluid supplied through therotary shaft to act in opposition to passive biasing means, such as aspring, located on the plate while the actuator and biasing means urgethe magnetron in opposed radial directions. The motion of the magnetronmay be linear on the plate or be pivoting about a pivot axis on theplate offset from the rotation axis of the plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a self-ionizing plasma(SIP) sputter reactor.

FIG. 2 is a schematic view of an embodiment of a centrifugal dualposition magnetron mechanism of the invention.

FIG. 3 is a schematic plan view of the dual position magnetron of FIG. 2in its outer position.

FIG. 4 is a schematic plan view of the magnetron of FIG. 2 in its innerposition.

FIG. 5 is a graph useful in explaining the kinetics of the movement ofthe dual position magnetron.

FIG. 6 is a cross-sectional view of an embodiment of the pivotingmechanism useful in a dual position magnetron.

FIG. 7 is an orthographic view of a first embodiment of a magnetronusable with the invention.

FIG. 8 is an orthographic view of a second embodiment of the magnetron.

FIG. 9 is a schematic plan view illustrating the two positions of apivoting, arc-shaped dual-position magnetron.

FIG. 10 is schematic plan view of a centrifugal slider for a dualposition magnetron of the invention.

FIG. 11 is a schematic cross-sectional view of a mechanism including anactuator for moving the radial position of a magnetron.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One general aspect of the invention comprising a two step processincluding alternating a first step of sputtering from one annular regionof a sputter target, particularly one composed of a refractory barriermaterial and typically used at least partially for reactive sputterdeposition and a second step of sputtering from at least a remainingportion of the sputter target. Both steps may use the same magnetron byshifting the radial position of the rotating magnetron between thesteps. The second step is particularly effective at cleaning the targetof redeposited material. Advantageously, in both steps the magnetron isrotated in the same direction about the target center.

For sputter depositing barriers into high aspect-ratio holes, the firststep is advantageously performed by scanning a relatively small andunbalanced magnetron near the peripheral edge of the target by rotatingthe magnetron about the central axis of the target. Then, the magnetronis moved towards the central axis of the target to scan the area closeto the target center and the magnetron is scanned in the same azimuthaldirection but at a different radius. With the magnetron located near thetarget center, it is rotated about the target center to provide moreuniform cleaning. Although it is typical to continue rotating themagnetron during its radial movement, both the edge sputtering and thecenter cleaning typically require much longer than a second so themagnetron is rotated many hundreds of revolutions at each of its twopositions.

The mechanism for moving the magnetron can assume many different forms.

One aspect of the invention includes a dual-position centrifugalpivoting magnetron assembly 80, illustrated in the orthographic view ofFIG. 2, which is switchable between two rotation radii dependent uponthe speed of rotation in a single rotation direction. The magnetronassembly 80 includes a bracket 82 that is fixed to the rotary driveshaft 62 rotating about the central rotation axis 60 of the reactor. Theend of one arm of the bracket 82 rotatably supports beneath it a pivotplate 84 through a pivot mechanism 86 that allows the pivot plate 84 topivot about a pivot axis 88. The pivot plate 84 supports a back plate90, which is composed of a ferromagnetic material to form the yoke ofthe magnetron 50. For structural purposes, the back plate 90 can beconsidered as part of the pivot plate 84 since they pivot together aboutthe pivot axis 88. One bushing 92 is affixed to the bracket 82 betweenthe rotation axis 60 and the pivot mechanism 86 and another bushing 94is fixed to a mount 95 on the back plate 90. The two ends of a tensionspring 96 are attached to the two bushings 92, 94. A first nylon stop100 is screwed to the pivot plate 84 on one side of the bracket 82 and asecond nylon stop 102 is screwed to the back plate 90 on the other sideof the bracket 82. Each stop 100, 102 includes a metal knob with athrough hole for the screw and a tubular soft, resilient nylon sheathfit on its outside to buffer the impact and shock of the sudden abutmentagainst the bracket 82. The spring 96 biases the side of the back plate90 with the second stop 102 towards the bracket 82 and hence biases themagnetron toward the rotation axis 60. However, pivoting in the inwarddirection is limited by the second stop 102 abutting and engaging thebracket 82. On the other hand, rotation of the drive shaft 62 exertssignificant centrifugal force on the heavy magnetron 50 and associatedelements and pushes the side of the back plate 90 with the second stop102 away from the bracket 82 in the radially outward direction away fromthe rotation axis 60. However, pivoting in the outward direction islimited by the first stop 100 abutting and engaging the bracket 82. Thespeed of rotation determines whether the inward spring biasing or theoutward centrifugal force prevails.

The distinguishing terminology is used that the rotary drive shaft 62rotates while the pivot plate 84 pivots although the two motions are insome sense equivalent. However, a more telling distinction between thetwo motions is that the magnetron 50 rotates at a constant but selectedradius from the rotation axis 60 for a large number of 360° revolutionsduring processing while the pivot plate 84 pivots over less than about90° in changing from one radial position to another. The terminology ofmechanical bias is understood to mean a predetermined limited force thatis applied to a body to urge it to move in the direction of the biasforce but the actual movement will typically depend upon countervailingforces opposing the movement.

A counterweight 104 is fixed to the other arm of the bracket 82 and isdesigned to have the same moment of inertia as the magnetron 50 andassociated elements to reduce vibration during rotation. However, sincethe moment of inertia of the magnetron 50 depends upon its radialposition, counterbalancing is improved if the counterweight is subjectto the same radial movement with respect to the rotation axis 60. Aposition flag 106, such as a magnet, is fixed to the backing plate 90,and a position sensor 108, such as a magnetic Hall sensor, illustratedin FIG. 1, is disposed in the roof over the rotating magnetron 50 toallow the controller 48 to determine the current radial position of themagnetron as the rotating magnet 106 either does or does not passbeneath the magnetic sensor 108.

In this embodiment of the invention, varying the rotation rate of therotary drive shaft 60 will either cause the centrifugal force toovercome the spring bias force to place the magnetron 50 at a first,radially outer position or cause the spring bias force to dominate toplace the magnetron 50 at a second, radially inner position. Themagnetron position illustrated in FIG. 2 is the OUT position for whichthe centrifugal force is greater. The OUT position is schematicallyillustrated in the plan view of FIG. 3, in which the pivot plate 84 isemphasized but simplified. As the rotary drive shaft 62 rotates in onerotational direction at the rate f_(OUT) that is sufficiently high, thecentrifugal force on the magnetron 50 is greater than the spring tensionand causes the pivot plate 84 and attached magnetron 50 to pivotoutwardly about the pivot axis 88 from the bracket 82 to the illustratedouter position. In contrast, in the IN position schematicallyillustrated in the plan view of FIG. 4, the rotary drive shaft 62rotates in the same rotational direction but at a sufficiently lowerrate f_(IN) that the spring tension is greater than the centrifugalforce so that the spring 96 pulls the pivot plate 84 and attachedmagnetron 50 to pivot inwardly about the pivot axis 88 towards thebracket 82 and to the illustrated inner position. Regardless of themagnetron position, the drive shaft 62 continues to rotate the magnetron50 about the central rotation axis 62 but the radial displacement of themagnetron 50 from the central axis 62 is greater in the outer positionthan in the inner position. Thereby, the magnetron 50 scans differentareas of the target when it is at different radial positions.

The kinetics of the magnetron movement will be explained with referenceto the graph of FIG. 5. Lines 110, 111, 112 plot the torque about thepivot axis 88 exerted on the pivot plate 84 by the centrifugal forceexerted primarily by the heavy magnetron 50 as a function of pivot anglebetween the pivot plate 84 and the bracket 82 for three values of rotaryshaft rotation rate, 50 rpm, 68 rpm, and 85 rpm. The pivot torque, whichis always in the outward direction in the illustrated configuration ofthe mechanism, increases with rotation rate and also increases withpivot angle as the pivot plate 84 moves the magnetron 50 outwardly fromthe rotation axis 62. Dotted line 113 plots the torque about the pivotaxis 88 exerted by the tension in the spring 96 as a function of pivotangle. The spring torque is always in the inner direction. The nettorque is the difference between the centrifugal torque and the springtorque and the sign of the difference determines whether the pivot plate82 pivots inwardly and outwardly. It is noted that both the centrifugaland spring torque are affected by the pivot angle, not only because ofthe change of bias and centrifugal force with changing spring length andmagnetron radius but also by the changing geometry of the torque armwith respect to the bracket's longitudinal axis. Bistable operation ispossible if the two rotation rates f_(IN) and f_(OUT), such as 50 rpmand 68 rpm in the graph, are chosen to completely bracket the springtorque 113 for all values of pivot angle between the two stops so thatfor rotation at the lower f_(IN) the magnetron is torqued all the way tothe inner stop and for rotation at the higher f_(OUT) the magnetron istorqued all the way to the outer stop.

The dynamics however are complicated by the torque exerted by therotational seal between the relatively rotating pivot plate 84 andbracket 82, which is frictional in nature and always impedes any motion.The seal torque is greatest when there is no relative movement andlessens once motion has begun. Assuming that the magnetron is in stableouter position 1 with the bracket 82 abutting the first stop 100 androtating at f_(OUT)=68 rpm, the rotation rate needs to be lowered belowthe balance point at which the centrifugal torque equals the springtorque by a further rotation decrement that overcomes the seal torque.The magnetron then is temporarily at position 2 at which it is at theouter position but rotating at f_(IN)=50 rpm. Thereafter the magnetronquickly moves inward to stable inner position 3 at which the magnetronis at the inner position with the bracket abutting the second stop 102and rotating at 50 rpm. If 50 rpm is below the break away point at whichthe seal torque is exceeded, then the magnetron begins moving toposition 3 before its rotation rate has reached 50 rpm. Similarly, whenit is desired to move outwardly from the stable position 3, the rotationrate must be raised sufficiently for the difference between thecentrifugal torque and spring torque to exceed the seal torque, therebyputting the magnetron temporarily at position 4 at which it is rotatingat the inner position at 68 rpm. The magnetron then quickly movesoutwardly to stable outer position 1.

The transition between the two positions can be speeded if the rotationrate is increased or decreased more than is required to just beginmovement. For example, when moving from the outer to inner position, therotation rate may be decreased from 68 rpm to significantly below 50 rpmto significantly increase the unbalanced torque. When the magnetron hasarrived at the inner position, the rotation rate can be decreased to 50rpm. Similarly, when moving from the inner to the outer position, therotation rate may be increased from 50 rpm to significantly above 68 rpmto initiate and complete the movement and thereafter may be decreased to68 rpm or perhaps even lower.

The magnetron movements have been described with reference to FIGS. 2,3, and 4 without regard to hydrodynamic effects, which may becomeimportant when the rotation rates are being changed and before theswirling motion of the cooling water has reached steady state generallyfollowing the rotating bracket 82 and magnetron 50. The rotationdirection indicated in FIGS. 3 and 4 acts to pull the magnetron 50through the viscous cooling water. The additional hydrodynamic forceupon increasing rotation rate acts to speed up the transition toward theouter position. In contrast, if the rotation direction is opposite thatillustrated, it pushes the magnetron 50 through the fluid. When therotation rate is decreased, the hydrodynamic force more quickly pressesthe magnetron 50 towards the center position.

The pivot mechanism must be designed in view of several requirements. Itis immersed in the cooling water. It must rotatably support a heavymagnetron with minimum droop and vibration, particularly sincemagnetrons may need to be rotated in close proximity to the back of thetarget. The pivoting mechanism should impose a minimum of seal torqueand other frictional effects to reduce the required difference inrotation rates. The pivot mechanism 86 illustrated in thecross-sectional view of FIG. 6 provides satisfactory performance. Atleast one screw fixes a spindle 114 to the pivot plate 84 in alignmentto the pivot axis 88. The inner races of a pair of sealed lubricatedbearings 115, 116 separated by a tubular spacer 117 are captured ontothe spindle 114 by a flange 118 screwed to the spindle 110. The innerraces of the bearings 115, 116 are captured on the bracket 82 by anannular preload spring 120 pressed against the axial end of the outerrace of the upper bearing 115 by a cap 122 fixed to the bracket 82 byscrews. The cavity containing the bearings 115, 116 is sealed on its twoends against the aqueous environment by a static O-ring seal 124 betweenthe bracket 82 and the cap 122 and by a dynamic seal 126 between thelower outer periphery of the spindle 114 and an inwardly extending lip128 of the bracket 82. To reduce the frictional seal torque, thediameter of the dynamic seal 126 should be minimized.

The nylon stops 100, 102 may provide insufficient shock resistance inview of the heavy magnetron and the fragility of the strong magneticmaterials such as NdFeB. Accordingly, it may be desirable to replace thenylon stops with spring-loaded shock absorbers similar to those used onautomobiles, which may be mounted either on the bracket or on the pivotplate and back plate to more smoothly engage stops on the other member.

A first preferred embodiment of an unbalanced, arc-shaped magnetron 130is illustrated in the orthographic view of FIG. 7 generally from thebottom of FIGS. 1 and 2. Twenty cylindrical permanent magnets 132 of afirst magnetic polarity along the cylindrical axis are arranged in aclosed outer band on the back plate 90, which serves as a magnetic yokeas well as a support fixed to the pivot plate 84. Ten magnets 134 of thesame design but inverted to provide the opposed second magnetic polarityare arranged in an arc shape within the outer band. The outer magneticband thus has a magnetic intensity that is greater than the magneticintensity of the inner magnetic arc by a factor of two so that themagnetron is unbalanced, as has been discussed in the aforecited Fupatent. However, to further emphasize ionized sputtering of the outerperiphery of the target and provide further magnetic side confinement,the unbalance may be greater on the arc-shaped side nearer the targetedge than on the opposed arc-shaped side nearer the center. The outermagnets 132 are covered by a band-shaped magnetic pole piece 136 and theinner magnets 134 are covered by an arc-shaped magnetic pole piece 138.A smooth convex side 140 of the band-shaped magnetic pole piece 136 isgenerally aligned with the outer periphery of the target when themagnetron 130 is in the outer position and rotates in that position nearthe target circumference. A similar smooth convex side exists in theback plate 90. On the other hand, a small concave side is located on theother side of the band-shaped magnetic pole piece 136. The concave sidemay be smoothly shaped or form a cusp. The pole pieces 136, 138 arearranged to have a nearly constant gap between them which generallydefines a region of magnetic field parallel to the inner face of thetarget and forms a plasma loop adjacent the front face of the target. Anon-magnetic separator plate 142 fixed between the back plate 90 and thepole pieces 136, 138 includes apertures in which the magnets 132, 134snugly fit, thereby aligning them. The operational center of themagnetron 130 is approximated by the center of gravity of the magnets132, 134, which is within the arc-shaped inner pole piece 138.

Although the magnetron 130 has provided satisfactory results, thekinematics of changing between the two magnetron positions are improvedif the center of gravity is shifted radially outwardly so that there isa larger change in the moment of inertia as the magnetron movesradially. A second embodiment of a magnetron 150 having an outwardlydisplaced center of gravity is illustrated in the exploded bottomorthographic view of FIG. 8. A non-magnetic alignment and weight block152, for example, of non-magnetic stainless steel, replaces theseparator plate 142 of FIG. 7. Dowel pins in the block 152 align it withthe back plate 90 and the pole pieces 136, 138. Screws 154 pass throughthe back plate 90 and block 152 and are threaded into the pole pieces136, 138 to hold the magnetron together.

The block 152 includes a weight portion 156 extending to the outerconvex portion 140 of the band-shaped pole piece 136, that is, radiallyoutward when the magnetron 150 is in the outer position. The weightportion 156 has a thickness substantially equal to the length of themagnets and is substantially continuous except for axial throughapertures 158 formed for all the unillustrated inner magnets and theunillustrated radially outer half of the outer magnets. The block 152also includes a semi-collar portion 160 having a reduced thickness andhaving apertures for the remaining ten outer magnets 132.

The arc-shaped magnetron 150 has a center of gravity that is closer tothe smooth convex edge 140 than the magnetron 130 of FIG. 7 and also hasa simpler design with fewer parts, which are easier to assemble. Theadded weight of about 2 kg for the weight portion 156 facilitatestransitions between the two stable positions. However, the added weightmust be rotatably supported on a movable cantilevered structure at twodifferent radial positions so droop and vibration present greaterproblems than the lighter design of FIG. 8. Further, the additionalweight increases the shock at the stops.

The arc-shaped magnetron 130 or 150 has an overall shape defined by theouter perimeter of the gap between two pole pieces 136, 138 that issubstantially longer in one direction than in the other. This shape isadvantageous for the two-step sputter deposition and cleaning process.As illustrated in the bottom plan view of FIG. 9, during the sputterdeposition, the arc-shaped magnetron 130 illustrated in solid lines isdisposed near an effective edge 170 of the target 16 with its longerdimension aligned along a circumference of the target 16 andperpendicular from a radius extending from the rotation axis 60 to thecenter of the magnetron and its shorter dimension is aligned generallyalong that target radius. In particular, the outer portion of the outerpole 136 in the sputter deposition may overlie the effective edge 170 ofthe target 16. A gap 172 between the two poles 136, 138 generallydefines a plasma loop inside the sputter chamber which producessputtering of the annular region of the target 16 adjacent the plasmaloop which extends radially inward from near the target edge 170. In oneembodiment of edge sputtering, the plasma tracks scans an annular bandthat extends in the outer half of the target radius and none of thetarget within the inner half is scanned.

During the sputter deposition, the RF source 28 strongly biases thewafer 18 to accelerate the sputtered ions deep within the highaspect-ratio holes being conformally coated with barrier material. Themagnetron 130 is relatively small but it is rotated about the targetcenter 60 to produce a relatively uniform sputter erosion in the annularband about the target center 60 including the magnetron gap 172. Itssmall size causes an average sputter rate in the annular band to berelatively small. However, the low deposition is acceptable for barrierlayers in high aspect-ratio holes, which need to be thin to leave thebulk of the hole for copper or other metallization.

At the beginning of the cleaning phase, the speed of rotation isdecreased and the magnetron 130 swings about the pivot point 88 to aposition illustrated by dotted lines closer to the target center 60. Inthis position, the long dimension of the magnetron 130 is aligned moreclosely to a radius of the target 16 than to a target circumference,thus allowing a larger radius of the target 16 to be cleaned. Asillustrated, the long dimension is inclined at about 45° from the radiusfrom the rotation axis 60 to the magnetron center, but the inclinationangle may advantageously be an angle between 0° and 60°. Alsoadvantageously, the magnetron gap 172 in the cleaning position extendsfrom the target center 60 to the gap 172, which defines the plasma loop,in the deposition position. As a result, all of the target radiallyinwards of the region sputtered in the deposition step is sputtered inthe cleaning step. During the cleaning step, the wafer bias may bereduced even to zero to minimize energetic ion flux at the waferpedestal.

Other types of biasing means than the tension spring 96 illustrated inFIG. 2 may be used. A compression spring can be substituted by moving itto the other side of the bracket 82. A leaf spring or spiral spring canbe used, for example, having one fixed near to the pivot axis and theother on the pivot plate 84 or back plate 90. A spring-wound wheelhaving an axis fixed to one member can be wrapped with a cable that hasits other end fixed to the other member. The biasing means describedhere are all passive though active biasing is possible.

In another embodiment of a centrifugal magnetron illustrated in the planview of FIG. 10, a linear slider mechanism 180 includes the bracket 82fixed to and rotating with the rotary drive shaft 62 about the targetcenter axis 60. A radially extending slot 182 is formed in the bracket82 to closely accommodate and align an oval-shaped support pier 184which can slide radially within the slot 182. The support pier 184supports the magnetron 50 beneath the bracket 82 while two support beams186 slidably support the support pier 184 on the upper surface of thebracket 82 through low-friction interfaces. An unillustratedcounterweight may be attached to the end of the bracket 82 opposite theslot 182, either fixed thereto or radially movable in correspondence tothe movement of the magnetron 50. Two springs 188, 190 are connected ontheir respective ends to a first cross-arm 192 fixed to the bracket 82and a second cross-arm 194 fixed to the support pier 184 to thereby biasthe magnetron 50 towards the target center 60. However, at asufficiently high rotation rate, the centrifugal force exerted on therotating magnetron 50 is sufficient to overcome the bias force and causethe magnetron into the illustrated outer position with the outer end ofthe support pier engaging an outer end 196 of the slot 162. But, at asufficiently low rotation rate, the bias force overcomes the centrifugalforce to cause the magnetron to move to its inner position with theinner end of the support pier 184 being stopped by an inner end 198 ofthe slot 182.

It is possible to design a centrifugally driven magnetron in which theouter position is favored by the biasing means and lower rotation ratesand the inner position is favored by the increased centrifugal force athigher rotation rates.

The stops provide definite bistable operation. It is however possible tomore finely balance the centrifugal force against the biasing means sothat the magnetron can be rotated at more than two radial positionswithout the use of intermediate stops.

Other position adjusting mechanism can be applied to the invention. Forexample, Fu et al. in U.S. Pat. No. 6,692,617 disclose an activelycontrolled adjusting mechanism 200, illustrated in the schematic crosssectional view of FIG. 11. A support plate 202 is fixed to the end ofthe rotary drive shaft 62 extending along target axis 60 to support themagnetron 50. A support pier 204 supporting the magnetron 50 on therotating support plate 202 slides in a radial slot 206 in the supportplate 202 that guides the support post 204 in the radial direction. Apneumatic or hydraulic actuator 208 having an actuator arm connected tothe support pier 204 is supported on the support plate 202 and ispowered through a fluid line 210 formed in the rotary shaft 62 andconnected through an unillustrated rotary coupling to a fluid powersource. A spring 210 or other biasing means connected between thesupport pier 204 and the support plate 202 may be used in opposition toa force applied to the support post 204 by the actuator 208. However, aseparate biasing means can be eliminated if the actuator 208 acts inopposition to centrifugal force. In any case, application of fluid forcethrough the actuator 208 can move the magnetron 50 in a radial directionof the rotating bracket 82 so that the magnetron can be placed at atleast two radial positions, one favoring sputtering of the edge, and theother favoring the center of the target 16. The actively controlledadjusting mechanism 200 allows the radial adjustment of the magnetron 50regardless of rotation rate. If the adjusting mechanism 200 has morethan two positions, the radial position of the magnetron 50 can be morefinely controlled.

One actuator-based design includes the pivoting arc-shaped magnetron ofFIG. 2 with approximately the same geometry but without the need forbistable modes. Instead, rather than relying upon changes in rotationrates to control the rotation radius, an externally controlled actuator,for example, connected between respective pivots on top of the stop 100and on the side of the bracket 82 opposite the pivot axis 88 andselectively supplied with pneumatic or hydraulic force through the fluidline 210 in the rotary shaft 62, as previously described with referenceto FIG. 11. The spring 96 may no longer be needed. However, if theactuator exerts force only upon extension, a spring may bias the backingplate 90 in the opposite direction, for example, by being connectedbetween the above mentioned pivots. However, centrifugal force along mayprovide the required biasing.

Other adjustment mechanisms are possible, for example, two coaxialshafts, one controlling the rotation and one the radial adjustment.Rosenstein's bi-directional centrifugal magnetron may also be used toalternate between sputter deposition and target cleaning althoughreversing directions impacts operational throughput.

The quick transition speeds of the uni-directional dual-positionmagnetron allow it to be used to clean the target for each wafer withoutseriously impacting throughput. The cleaning may be performed after theprocessed production wafer has been removed from the reactor and beforea new production wafer is inserted into the reactor. On the other hand,cleaning for each wafer requires only a very short sputter period ofsputtering material already in the barrier As a result, the cleaning maybe performed in situ while the production wafer is in the reactor. Inparticular, for a bilayer barrier layer, for example, of TaN/Ta, thesputtering may begin with a short, for example 2 or 3 seconds, period ofreactive sputtering of TaN in the presence of nitrogen with themagnetron positioned near the target center because of low rotationrate. The short initial inner sputtering should be sufficient to preventTaN from accumulating on the target center after many process cycles.Thereafter, increased rotation rate may move the magnetron nearer to thetarget periphery while the sputtering of TaN continues. The final Tasputtering may be performed with the magnetron positioned nearer to thetarget periphery.

Although the invention has been described for varying the magnetronposition between the steps of sputter deposition and target cleaning,the magnetron can be moved for other purposes, for example, igniting theplasma or changing to sputter etching of the wafer. Gung et al. describea sputter reactor configurable for different modes of operationincluding differentially controllable quadruple electromagnet array inprovisional application 60/574,905, filed May 26, 2004 and inconcurrently filed U.S. patent application Ser. No. 10/950,349 entitledVARIABLE QUADRUPLE ELECTROMAGNET ARRAY IN PLASMA PROCESSING,incorporated herein by reference in its entirety. The invention is alsoadvantageously practiced with a mechanism for varying the spacingbetween the target and the magnetron, as disclosed by Subramani et al.in provisional application 60/555,992, filed Mar. 24, 2004. Hong et al.disclose a more general mechanism and process in U.S. Patent applicationSer. No. 10/942,273, filed Sep. 16, 2004 and entitled MECHANISM FORVARYING THE SPACING BETWEEN SPUTTER MAGNETRON AND TARGET, incorporatedherein by reference in its entirety.

Although the invention has been described in terms of sputtering arefractory barrier metal, particularly tantalum, and its nitride, theinvention may be applied to other barrier metals, such as titanium andtungsten, and also to metals used in siliciding, such as cobalt, nickel,and molybdenum. The invention may also be applied to metallizationmetals and their seed layers, particularly aluminum, which is subject toflaking redeposition, and also to copper, for which different sputteringcharacteristics may be desired in a multi-step process. The flexibilityafforded by a two-position magnetron may be used in sputtering othertypes of materials, including RF sputtering of metals or insulators.

The invention thus provides additional control of sputtering and targetcleaning and of controllably varying sputtering characteristics withfairly minor changes in the magnetron scanning mechanism that can beeasily incorporated into existing reactor designs.

1. A method of sputtering in a plasma sputter reactor arranged about acentral axis and having a target in opposition to a pedestal supportinga substrate to be sputter coated with a material of said target,including the steps of: a first step of rotating a nested unbalancedmagnetron at a first radius about said central axis to sputter depositmaterial into holes formed in said substrate; and a second step ofrotating said magnetron at a second radius less than said first radiusabout said central axis to clean material from said target depositedthereon in said first step.
 2. The method of claim 1, wherein a plasmaloop of said magnetron located at said second radius passes through saidcentral axis.
 3. The method of claim 1, wherein said target comprises arefractory barrier material.
 4. The method of claim 3, wherein saidfirst step includes flowing nitrogen into said chamber, no nitrogenbeing flowed into said chamber during said second step.
 5. The method ofclaim 1, wherein during said first step said pedestal is biased with afirst level of RF power and during said second step said pedestal iseither unbiased or is biased with a second level of RF powersubstantially less than said first level.
 6. The method of claim 1,wherein said magnetron is rotated in a same direction about said centralaxis during said first and second steps.
 7. A multi-position magnetron,comprising: a rotary shaft extending along a rotation axis; a platefixed to said rotary shaft; an arm pivotable on said plate about a pivotaxis displaced from said rotation axis rotary shaft and pivotingindependently of a rotation angle of said rotary shaft; a magnetronfixed to said plate, a center of said magnetron being offset from saidpivot axis, whereby dependent upon pivoting of said arm, said rotaryshaft causes said center to rotate in a single direction about saidrotation axis at a selected one of a plurality of radii from saidrotation axis.
 8. The magnetron of claim 7: wherein said magnetron isnon-circularly symmetric and has a long dimension and a short dimension;wherein, when said center rotates at a first radius, said long dimensionextends perpendicularly to a line extending from said rotation axis tosaid center; and wherein, when said center rotates at a second radiusless than said first radius, said long dimension is inclined at no morethan 60° from a line extending from said rotation axis to said center.9. The magnetron of claim 8: wherein said magnetron is nested andincludes an inner pole of first magnetic polarity, an outer pole ofsecond magnetic polarity opposed said first magnetic polarity,surrounding said inner pole, and separated therefrom by an annular gap;wherein, when said center rotates at said first radius, said annular gapscans an annular band separated from said rotation axis; and wherein,when said center rotates at said second radius, said annular gap scans acircular region including said rotation axis and extending radiallyoutwardly at least to said annular band.
 10. The magnetron of claim 7,further comprising an externally controlled actuator connected betweensaid plate and said arm to control said pivoting of said arm about saidpivot axis.
 11. The magnetron of claim 10, wherein said rotary shaftincludes a fluid line connected to said actuator.
 12. A multi-positionmagnetron for a plasma sputter reactor, comprising: a bracket fixable toa rotary drive shaft extending along a central axis and rotatabletherewith; a pivoting mechanism fixed on said bracket and pivotallysupporting a pivot arm about a pivot axis; a magnetron supported on saidpivot arm away from said pivot axis; and biasing means coupling saidbracket and said pivot arm acting in opposition to centrifugal forceexerted on said magnetron by rotation of said rotary drive shaft. 13.The magnetron of claim 12, wherein said biasing means comprises aspring.
 14. The magnetron of claim 13, wherein said spring is a tensionspring having one end fixed to said bracket and another end fixed tosaid pivot plate.
 15. The magnetron of claim 12, further comprisingfirst and second stop means preventing further pivoting of said pivotplate relative to said bracket in respective opposed directions aboutsaid pivot axis.
 16. The magnetron of claim 15, wherein at a firstrotation rate of said rotary shaft said magnetron is disposed at a firstradius from said central axis and at a second rotation rate less thansaid first rotation rate of said rotary shaft said magnetron is disposedat a second radius less than said first radius from said central axis.17. The magnetron of claim 12, wherein said magnetron is an unbalancedmagnetron having a first pole having a first magnetic polarity alongsaid central axis and a second pole of greater magnetic strength thansaid first pole, surrounding said first pole, and having a secondmagnetic polarity opposite said first magnetic polarity.
 18. Themagnetron of claim 12, wherein, when said magnetron is disposed at saidfirst radius, said central axis passes outside of a periphery of saidsecond pole and, when said magnetron is disposed at said second radius,said central axis passes inside of said periphery of said second pole.19. A multi-position magnetron for a plasma sputter reactor, comprising:a bracket fixable to a rotary drive shaft extending along a central axisand rotatable therewith; a magnetron; a support mechanism supportingsaid magnetron on said bracket and allowing radial movement of saidmagnetron about said central axis; and biasing means coupling saidbracket and said support mechanism acting in opposition to centrifugalforce exerted on said magnetron by rotation of said rotary drive shaft.20. The magnetron of claim 19, wherein said biasing means comprises atleast one spring.
 21. The magnetron of claim 20, wherein said supportmechanism comprises a pivot arm pivotally supported on said bracket andsupporting said magnetron and said at least one spring is coupledbetween said bracket and said pivot arm.
 22. The magnetron of claim 20,wherein said support mechanism comprise a support member fitted in andsliding along a slot formed in said bracket and said at least one springis coupled between said bracket and said support member.
 23. A dualposition magnetron for scanning a target having a central axis in aplasma sputter reactor, comprising: a bracket affixable to a rotaryshaft rotatable about said central axis; a pivot mechanism fixed on saidbracket and pivotally supporting a pivot plate about a pivot axis; amagnetron fixed to said pivot plate away from said pivot axis; a springcoupling said bracket and said pivot plate and urging said magnetrontoward said central axis in opposition to centrifugal force exerted onsaid magnetron by rotation of said rotary shaft.
 24. The magnetron ofclaim 23, further comprising: a first mechanical stop preventing furtherpivoting of said pivot plate with respect to said bracket in a firstpivoting direction about said pivot axis; and a second mechanical stoppreventing further pivoting of said pivot plate with respect to saidbracket in a second pivoting direction opposite said first pivotingdirection.
 25. The magnetron of claim 24, wherein said first and secondmechanical stops are fixed to said pivot plate and engage said bracketupon predetermined pivoting of said pivot plate about said pivot axis.26. The magnetron of claim 23, wherein said pivoting mechanism includesbearings sealed against water surrounding said magnetron.
 27. Amagnetron sputter reactor, comprising: a vacuum chamber arranged arounda central axis; a sputter target sealing one end of the vacuum chamber;a pedestal for supporting a substrate in opposition to a front side ofsaid target; a rotary drive shaft extending along and rotatable aboutsaid central axis; a bracket fixed to said rotary drive shaft androtatable therewith; a pivot plate; a pivoting mechanism fixed to saidbracket and pivotally supporting said pivot plate about a pivot axisdisplaced from said central axis; a magnetron supported on said pivotplate adjacent to a back side of said target; and a spring coupling saidbracket and said pivot plate and biasing said magnetron to pivot aboutsaid pivot axis towards said central axis.
 28. The reactor of claim 27,wherein rotation of said rotary drive shaft urges said magnetron awayfrom said central axis.
 29. The reactor of claim 27, further comprisingfirst and second mechanical stops preventing further pivoting of saidpivot plate relative to said bracket in respective opposed pivotdirections about said pivot axis.
 30. The reactor of claim 27, furthercomprising a liquid enclosure for a chilling liquid to cool said target,wherein said bracket, pivoting mechanism, magnetron, and spring aredisposed inside said liquid enclosure.
 31. A method of operating amagnetron sputter reactor comprising a rotary drive shaft supporting amagnetron pivotally supported on a bracket connected to said rotarydrive shaft and biased towards said central axis, said method comprisingthe steps of: a first step of rotating the rotary drive shaft in a firstdirection about said central axis at a first rotation rate to cause saidmagnetron to move to a first position at which it rotates at a firstradius about said central axis; and a second step of rotating the rotarydrive shaft in said first direction at a second rotation rate differentthan said first rotation rate to cause said magnetron to move to asecond position away from said first position at which it rotates at asecond radius different than said first radius about said central axis.32. The method of claim 31, wherein said first rotation rate is greaterthan said second rotation rate and said first radius is greater thansaid second radius.
 33. The method of claim 32, further comprising afirst transition step in changing from said first rotation step to saidsecond rotation step of rotating said rotary drive shaft at a thirdrotation rate less than said second rotation rate.
 34. The method ofclaim 31, wherein said magnetron is mechanically and passively biasedtowards said central axis.
 35. The method of claim 31, whereincentrifugal force produced by rotation of said rotary urges saidmagnetron away from said central axis.
 36. The method of claim 1,wherein during said first step said pedestal is biased with a firstlevel of RF power and during said second step said pedestal is with alevel of power within a range of zero to a second level of RF powersubstantially less than said first level.
 37. The method of claim 1,wherein said first and second steps both including rotating a rotaryshaft which supports the magnetron and extends along said central axisduring the first and second steps, wherein a point within the magnetronis displaced at said first radius from said central axis during saidfirst step and at said second radius during said second step.
 38. Amulti-position magnetron for a plasma sputter reactor, comprising: abracket fixable to a rotary drive shaft extending along a central axisof the chamber and rotatable therewith; a pivoting mechanism fixed onsaid bracket and pivotally supporting a pivot arm about a pivot axis,whereby a variable pivot angle is provided between the bracket and thepivot arm; and a magnetron supported on said pivot arm away from saidpivot axis, wherein a point on the magnetron is displaced from thecentral axis by a selected radius determined by the variable pivot angleduring a complete rotation of the rotary drive shaft about the centralaxis in a predetermined rotation direction.
 39. The magnetron of claim38, further comprising centrifugal means for determining a value of thevariable pivot angle.
 40. The magnetron of claim 38, further comprisingan externally controlled actuator acting between the bracket and thepivot arm for determining a value of the variable pivot angle.